Two well-known techniques have been suggested for use in achieving low temperature or cryogenic operation, particularly using helium as a fluid, for example. One approach, often referred to as a Collins cycle (alternatively such approach is often referred to as a multi-stage Claude cycle), is described in one of its basic forms in U.S. Pat. No. 2,458,594 issued on Jan. 11, 1949 to S. C. Collins. The Collins cycle is used to provide refrigeration or liquefaction at "liquid-helium" temperatures. The Claude cycle is used to provide refrigeration or liquefication at higher temperatures using fluids such as nitrogen. Improvements and modifications to the basic technique have also been described in U.S. Pat. Nos. 2,607,322 and 3,438,220, for example, issued to S. C. Collins on Aug. 19, 1952 and Apr. 15, 1969, respectively.
In such approach, high pressure fluid from a compressor is passed through a heat exchanger and introduced, via a high pressure valve, into an expansion engine comprising a chamber having a moveable member such as a piston positioned therein When the fluid is so introduced, the piston moves within the chamber to form an expansion volume, the expansion of the fluid causing the heat energy to be transferred therefrom via the performance of mechanical work, as on a crankshaft, for example, connected to the piston. In the expansion operation, the temperature and pressure of th fluid are reduced considerably. The fluid is then conveyed via a low pressure valve from the expansion volume to a space to be cooled, for example, and then back to the compressor in a countercurrent flow through the heat exchanger.
The pressure of the fluid as it flows in both direction through the heat exchanger is maintained substantially constant both during the high pressure flow from the input side to the expansion engine and during the low pressure flow from the expansion engine to the output side. Further, during the high and low pressure flows there is substantially little or no intermediate storage of heat energy involved. Moreover, in specific embodiments of the technique, the high pressure valve and the low pressure valve are both located in the apparatus at the low temperature region thereof.
While the Collins cycle technique is effective when used for relatively large-scale production of low temperature helium, for example, it has been found to be difficult to scale down the apparatus size when a smaller system is required and still retain the low temperature effectiveness thereof.
Another approach used in the art to achieve low temperature operation is often referred to as the Gifford-McMahon cycle technique, an approach which has sometimes been proposed as effective when used for such smaller scale systems. The Gifford-McMahon cycle is commonly used in single- and multiple-stage configurations. A multi-stage Gifford-McMahon cycle, however, is not capable of producing liquid-helium temperatures with conventional regenerator materials. Refrigeration in a Gifford-McMahon operation results from a difference in enthalpy rates between the entering high pressure stream and the exiting low pressure stream. A basic description of the Gifford-McMahon operation is set forth in U.S. Pat. No. 3,045,436, issued on Jul. 24, 1962 to W. E. Gifford and H. O. McMahon. Other apparatus using Giffin-McMahon principles of operation are also described, for example, in U.S. Pat. Nos. 3,119,237 and 3,421,331 issued on Jan. 28, 1964 and Jan. 14, 1969 to W. E. Gifford and to J. E. Webb, respectively.
In such systems, no heat energy is transferred from the expanding fluid through the performance of mechanical work external to the refrigerator. While a moveable displacer element is periodically moved within the apparatus to provide for an expansion chamber, such element is not arranged so as to produce mechanical energy exchange. Rather, multiple confined fluid volumes are balanced so as to act in conjunction with one another so that compression and expansion are selectively controlled using inlet and exhaust valves at room temperatures and a net refrigeration is produced at one or more points in the system.
In such an approach, the confined fluid volumes on either end of the displacer are connected by a heat exchange passage called a thermal regenerator. Thus, the regenerator undergoes the same pressure cycling as the confined fluid volumes In this configuration, the heat energy is normally fully stored for a half cycle in the regenerator matrix, which requires the regenerator matrix to have a large heat capacity. In totally regenerative cycles, such as in the Gifford-McMahon approach, the pressure ratio is effectively limited by the gas volume in the regenerator, which must be large enough so that the low-pressure flow pressure drop through the regenerator matrix is not excessive.
Common regenerator materials have a heat capacity that diminishes at very low temperatures For this reason, the Gifford-McMahon cycle alone is not capable of producing cooling at liquid-helium temperatures, even when multiple stages are used. To reach liquid-helium temperatures, a second thermodynamic cycle such as a Joule-Thomson cycle must be used in parallel with the Gifford-McMahon cycle. The Joule-Thomson cycle consists of a pre-cooling counterflow heat exchanger and an expansion valve (commonly referred to a the Joule-Thomson expansion valve). Neither the Gifford-McMahon nor the Joule-Thomson cycles are capable of reaching liquid-helium temperatures independently. The Gifford-McMahon stages provide precooling of the helium gas in the counterflow heat exchanger of the Joule-Thomson cycle in preparation to expand the gas over the Joule-Thomson expansion valve. This combined cycle configuration is capable of producing cooling at liquid-helium temperatures. However, integrating these two cycle configurations is undesirable for two reasons. First, mechanically combining the two configurations is somewhat cumbersome, especially during manufacture. Second, the optimal mean cycle pressures and pressure ratios for the two cycles are not compatible, which requires a special compressor configuration. It is desirable to combine the advantages of regenerative heat exchange, as in the Gifford-McMahon cycle with the advantages of counterflow heat exchange as in the Collins approach in a single package that uses, for example, a displacer type expansion engine, particularly for designing a relatively scaled-down refrigeration apparatus which may be required in many applications.